Water Journal : Water Journal September 2012-1
refereed paper membrane technology water SEPTEMBER 2012 71 During commissioning, the feedback control implemented for sodium hydroxide dosing was found to be ineffective and resulted in overdosing. The dosing was run in manual mode until the problem was rectified. It was eventually found that the installed control logic was not consistent with the logic documented in the Automatic Control and Monitoring Manual (ACMM). Detailed Site Acceptance Testing (SAT) had not been conducted for this packaged plant because the same logic had been used on a number of other plants previously. Experiences with the Paxton upgrade showed that it is always vitally important to conduct detailed SAT in order to prevent serious control problems at comissioning. This upgrade also provided information on the performance and optimisation of the MBR process. Operational optimisation following commissioning has maintained low effluent TSS and BOD5 concentrations with less energy and chemicals. Although operation of the upgraded plant initially required more operator input than expected, changes to control have simplified the operation while still providing the flexibility and reliability that is required. Some equipment-related issues that were encountered during the project are further described in the following text. Screenings bypass occurred during commissioning due to the seals between the screen and the housing not being correctly installed. This issue also occurred on another MBR process commissioned by the Alliance three months after the Paxton plant was commissioned. A temporary modification was made until a permanent solution could be provided by the supplier. A photo of this patch is shown in Figure 8. Any screenings that bypass could damage the membranes. If the screens do have bypass, it is important to ensure that the entire contents of the bioreactor are recirculated through the screens again to prevent screenings entering the MBR. In-line strainers (2mm aperture) on the recirculation line from the bioreactor to the inlet works were found to frequently block up (Figure 9). These strainers had to be cleaned hourly while the plant was operating to maintain sufficient recirculation flow. The 2mm strainers were replaced with 6mm aperture strainers; however, the blocking persisted. The upgrade contractor subsequently made a decision to remove the strainers completely. 0 10 20 30 40 50 60 70 80 90 r e t f A e r o f e B BOD5 Concentraton (mg/L) Eﬄuent BOD5 Concentraton Boxes represent the 5th and 95th percentles, the whiskers represent the minimum and maximum values Figure 4. BOD5 results before and after the upgrade. 0 10 20 30 40 50 60 70 80 90 r e t f A e r o f e B Total Suspended Solids Concentraton (mg/L) Eﬄuent Total Suspended Solids Concentraton Boxes represent the 5th and 95th percentles, the whiskers represent the minimum and maximum values Figure 5. TSS results before and after the upgrade. 0 5 10 15 20 25 r e t f A e r o f e B Total Nitrogen Concentraton (mg/L) Eﬄuent Total Nitrogen Concentratons Boxes represent the 5th and 95th percentles, the whiskers represent the minimum and maximum values Figure 6. Total nitrogen results before and after the upgrade. 0 2 4 6 8 10 12 14 16 18 r e t f A e r o f e B Total Phosphorus Concentraton (mg/L) Eﬄuent Total Phosphorus Concentratons Boxes represent the 5th and 95th percentles, the whiskers represent the minimum and maximum values Figure 7. Total phosphorus results before and after the upgrade. Figure 8. Drum screen seal damage and temporary patch. Figure 9. Blocked strainers (left) and cleaning a strainer (right).
Water Journal November 2012-1
Water Journal August 2012